Amplifier circuit and operation method thereof

ABSTRACT

Disclosed is an amplifier circuit capable of achieving high efficiency at back off power while maintaining high output power when an amplifier of a driving stage is saturated in a multistage amplifier in which a plurality of amplifiers are connected in series to each other. In the amplifier circuit, at least two amplifiers including a first amplifier and a second amplifier, the first amplifier preceding the second the first amplifier, are connected in series to each other, the second amplifier changes input impedance according to output power from the first amplifier, and an impedance adjusting unit for adjusting output load impedance of the first amplifier is disposed between the first amplifier and the second amplifier, wherein the impedance adjusting unit optimizes the output load impedance of the first amplifier according to a change of input impedance of the second amplifier.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority under 35 U.S.C. §365 toInternational Patent Application No. PCT/KR2013/010130 filed Nov. 8,2013, entitled “AMPLIFIER CIRCUIT AND OPERATION METHOD THEREOF”, and,through International Patent Application No. PCT/KR2013/010130, toJapanese Application No. 2012-268706 filed Dec. 7, 2012 and KoreanPatent Application No. 10-2013-0131264 filed Oct. 31, 2013, each ofwhich are incorporated herein by reference into the present disclosureas if fully set forth herein.

TECHNICAL FIELD

The present disclosure relates generally to an amplifier circuit and acommunication apparatus.

BACKGROUND ART

Lately, with development of high-speed, large-capacity wirelesscommunication schemes represented by Long Term Evolution (LTE), a needfor modulation signals having a great Peak to Average Power Ratio (PAPR)are increasing. Generally, in an amplifier for wireless communication,signals having a great PAPR require high consumption power, and causedeterioration of operation efficiency. In order to overcome theproblems, a Doherty amplifier capable of performing high-efficientoperations over a wide output range is used.

The Doherty amplifier generally includes a carrier amp and a peak amp.The carrier amp amplifies signals unconditionally, whereas the peak ampamplifies signals having higher power than specific power.

DISCLOSURE OF INVENTION Technical Problem

When power is low, only the carrier amp biased to a class AB amplifiespower, and the peak amp biased to a class C amplifies no power. Whenpower is high, both the carrier amp and the peak amp amplify power. TheDoherty amplifier can increase efficiency at back off power (averageoutput power for modulation signals) while maintaining high maximumoutput power, using a change of output power impedance of the carrieramp depending on the operation state of the peak amp and power combiningof the carrier amp and the peak amp.

The Doherty amplifier is widely used in wireless base stations. Lately,studies into using an amplifier based on the Doherty configuration dueto its conversion to other modes in mobile terminals have beenconducted. Also, studies into a broad-band Doherty amplifier areunderway.

By configuring an amplifier of a final stage as a Doherty amplifier in amultistage amplifier in which a plurality of amplifiers are connected inseries to each other, it is possible to achieve high efficiency at backoff power while maintaining high output power upon saturation. However,when an amplifier of a final stage of a multistage amplifier isconfigured as a Doherty amplifier, impedance matching between theDoherty amplifier and a driving amplifier provided at a stage precedingthe Doherty amplifier is done at 50_(Ω) or at arbitrary fixed impedance(in the following description, impedance matching is done at 50_(Ω)). Inthis case, increasing output power upon saturation of the drivingamplifier reduces efficiency at back off power, and increasingefficiency at back off power reduces output power upon saturation.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

Solution to Problem

Aspects of the present disclosure are to address at least theabove-mentioned problems and/or disadvantages and to provide at leastthe advantages described below. Accordingly, an aspect of the presentdisclosure is to provide an amplifier circuit and a communicationapparatus capable of achieving high efficiency at back off power whilemaintaining high output power when an amplifier of a driving stage issaturated in a multistage amplifier in which a plurality of amplifiersare connected in series to each other.

In accordance with an aspect of the present disclosure, there isprovided an amplifier circuit including: at least two amplifiersincluding a first amplifier and a second amplifier, the first amplifierpreceding the second amplifier; and an impedance adjusting unit disposedbetween the first amplifier and the second amplifier, and configured toadjust output load impedance of the first amplifier, wherein the firstamplifier and the second amplifier are connected in series to eachother, the second amplifier changes input impedance according to outputpower from the first amplifier, and the impedance adjusting unit adjuststhe output load impedance of the first amplifier according to a changeof input impedance of the second amplifier.

The impedance adjusting unit may include a matching circuit configuredto match the output load impedance of the first amplifier with the inputimpedance of the second amplifier. The impedance adjusting unit mayinclude a phase adjusting unit configured to adjust a phase of a signaloutput from the matching circuit.

The second amplifier may be a Doherty amplifier.

The second amplifier may be an envelope tracking amplifier.

In accordance with another aspect of the present disclosure, there isprovided an operation method of an amplifier circuit, the operationmethod including: at an impedance adjusting unit disposed between afirst amplifier and a second amplifier wherein the first amplifierprecedes the second amplifier, adjusting output load impedance of thefirst amplifier, wherein the first amplifier and the second amplifierare connected in series to each other, the second amplifier changesinput impedance according to output power from the first amplifier, andthe impedance adjusting unit adjusts output load impedance of the firstamplifier according to a change of input impedance of the secondamplifier.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the disclosure.

BRIEF DESCRIPTION OF DRAWINGS

The above and other aspects, features and advantages of certainexemplary embodiments of the present disclosure will be more apparentfrom the following description taken in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram illustrating an example of a functionconfiguration of an amplifier circuit according to an embodiment of thepresent disclosure;

FIG. 2 is a circuit diagram illustrating an example of a circuitconfiguration of an amplifier circuit according to an embodiment of thepresent disclosure;

FIG. 3 is a view for describing a change of input impedance of a Dohertyamplifier;

FIG. 4 is a view for describing a change of output load impedance of adriving amplifier;

FIG. 5 is a graph showing high-frequency characteristics of a drivingamplifier included in an amplifier circuit according to an embodiment ofthe present disclosure;

FIG. 6 is a block diagram illustrating an example of a functionconfiguration of a communication apparatus including an amplifiercircuit, according to an embodiment of the present disclosure; and

FIG. 7 is a flowchart of an operation method of an amplifier circuit,according to an embodiment of the present disclosure.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

MODE FOR THE INVENTION

The following description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the disclosure as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding, but these are to be regarded as merely exemplary.Accordingly, those of ordinary skill in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of thedisclosure. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but are merely used by theinventor to enable a clear and consistent understanding of thedisclosure. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent disclosure is provided for illustration purposes only and notfor the purpose of limiting the disclosure as defined by the appendedclaims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present disclosure. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise.

Hereinafter, an example of a function configuration of an amplifieraccording to an embodiment of the present disclosure will be described.FIG. 1 is a block diagram illustrating an example of a functionconfiguration of an amplifier circuit according to an embodiment of thepresent disclosure.

Hereinafter, an example of a function configuration of an amplifiercircuit according to an embodiment of the present disclosure will bedescribed with reference to FIG. 1.

Referring to FIG. 1, an amplifier circuit 100 according to an embodimentof the present disclosure may include a driving amplifier 110, animpedance adjusting unit 120, and a Doherty amplifier 130. The drivingamplifier 110 may amplify a signal received by the amplifier circuit100. The driving amplifier 110 may transfer the amplified signal to theDoherty amplifier 130 through the impedance adjusting unit 120.

The Doherty amplifier 130 may amplify the signal transferred from thedriving amplifier 110 through the impedance adjusting unit 120. TheDoherty amplifier 130 may include a carrier amp and a peak amp, and adetailed configuration of the Doherty amplifier 130 will be describedlater. The carrier amp amplifies signals unconditionally, and the peakamp amplifies signals having higher power than predetermined power.

The amplifier circuit 100 may be a multistage amplifier in which aplurality of amplifiers are connected in series to each other. In theamplifier circuit 100, an amplifier of a final stage may be configuredas the Doherty amplifier 130 as illustrated in FIG. 1. Ω when aplurality of amplifiers are connected in series to each other and anamplifier of a final stage is configured as the Doherty amplifier 130 asillustrated in FIG. 1, increasing output power upon saturation of thedriving amplifier 110 reduces efficiency at back off power, andincreasing efficiency at back off power reduces output power uponsaturation.

This is because when impedance matching is done at 50Ω, output loadimpedance from the driving amplifier 110 does not properly use a changeof input impedance of the Doherty amplifier 130.

Accordingly, the impedance adjusting unit 120 is disposed between thedriving amplifier 110 of a driving stage and the Doherty amplifier 130of a final stage. The impedance adjusting unit 120 may adjust outputload impedance of the driving amplifier 110 matching with inputimpedance of the Doherty amplifier 130. More specifically, the impedanceadjusting unit 120 may adjust output load impedance of the drivingamplifier 110 such that high-power load matching is done when high poweris output and high-efficient load matching is done at back off power.

As such, by disposing the impedance adjusting unit 120 between thedriving amplifier 110 of the driving stage and the Doherty amplifier 130of the final stage, and matching output load impedance of the drivingamplifier 110 using the impedance adjusting unit 120, it is possible toachieve high efficiency at back off power while maintaining high outputpower upon saturation of the driving amplifier 110.

An example of a function configuration of the amplifier circuit 100according to an embodiment of the present disclosure has been describedwith reference to FIG. 1. In FIG. 1, only two amplifiers of the drivingamplifier 110 and the Doherty amplifier 130 are illustrated, however,the present disclosure is not limited to this. For example, according toanother embodiment of the present disclosure, a plurality of amplifiersmay be disposed at the driving stage, and a Doherty amplifier may bedisposed at the final stage. Hereinafter, an example of a circuitconfiguration of the amplifier circuit 100 according to an embodiment ofthe present disclosure will be described.

FIG. 2 is a circuit diagram illustrating an example of a circuitconfiguration of the amplifier circuit 100 according to an embodiment ofthe present disclosure.

Hereinafter, an example of a circuit configuration of the amplifiercircuit 100 according to an embodiment of the present disclosure will bedescribed with reference to FIG. 2.

Referring to FIG. 2, the amplifier circuit 100 may include a drivingamplifier 110, an impedance adjusting unit 120, and a Doherty amplifier130. The impedance adjusting unit 120 may include a matching circuit 121and a phase adjusting unit 122. The Doherty amplifier 130 may include apower divider 131, a carrier amp 132, a peak amp 133, and impedancetransformers 134 and 135. Power is combined at a connection of theimpedance transformers 134 and 135.

The driving amplifier 110 may amplify a signal received by the amplifiercircuit 100, and transfer the amplified signal to the Doherty amplifier130 through the matching circuit 121 and the phase adjusting unit 122.

The matching circuit 121 may match impedance of the signal amplified bythe driving amplifier 110 with input impedance of the Doherty amplifier130. The matching circuit 121 may be configured as a combination of acoil and a condenser. The matching circuit 121 may transfer the signalsubject to the impedance matching to the phase adjusting unit 122. Thephase adjusting unit 122 may adjust the phase of the received signal,and transfer the signal whose phase has been adjusted to the powerdivider 131.

Input impedance of the Doherty amplifier 130 may vary depending on amagnitude of power, and output load impedance of the driving amplifier110 may be optimized by the matching circuit 121 and the phase adjustingunit 122 according to a magnitude of power. More specifically, thematching circuit 121 and the phase adjusting unit 122 may adjust outputload impedance of the driving amplifier 110 such that high-power loadmatching is done when high power is output from the amplifier circuit100, and high-efficient load matching is done at back off power.

The power divider 131 may divide the signal transferred from the phaseadjusting unit 122 into a first signal and a second signal, and transferthe first signal to the carrier amp 132 and the second signal to thepeak amp 133.

The carrier amp 132 may amplify the first signal transferred from thepower divider 131. The carrier amp 132 may be an amp biased to operatein a class B, a class AB, or a class A, and amplify the first signalunconditionally. The carrier amp 132 may transfer the amplified signalto the impedance transformer 134.

The peak amp 133 may amplify the second signal transferred from thepower divider 131. The peak amp 133 is biased to operate in a class C,and may amplify the second signal when the second signal has higherpower than predetermined power. The peak amp 133 may transfer theamplified second signal to the impedance transformer 135.

The impedance transformer 134 may transform impedance of the signalamplified by the carrier amp 132, and may be λ/4 transformer. Theimpedance transformer 134 may transfer the signal whose impedance hasbeen transformed to the impedance transformer 135.

The impedance transformer 135 may transform impedance of the signalamplified by the peak amp 133 and impedance of the signal whoseimpedance has been transformed by the impedance transformer 134. Theimpedance transformer 135 may also be a λ/4 transformer.

The Doherty amplifier 130 may be an inverted Doherty amplifier in whicha carrier amp and a peak amp are arranged at inverted positions to thoseillustrated in FIG. 2. Also, the Doherty amplifier 130 may be aseries-connected Doherty amplifier in which a carrier amp and a peak ampare connected in series to each other.

Hereinafter, a change of input impedance of the Doherty amplifier 130will be described. As described above, the peak amp 133 is biased tooperate in the class C, and the operation state of the peak amp 133 mayvary depending on input power. Accordingly, input impedance of theDoherty amplifier 130 may greatly depend on the operation state of thepeak amp 133 according to input power.

FIG. 3 is a view for describing a change of input impedance of theDoherty amplifier 130.

A point denoted by a reference number 301 in FIG. 3 represents anexample of input impedance at low power, and a point denoted by areference number 302 in FIG. 3 represents an example of input impedanceat high power. As illustrated in FIG. 3, input impedance of the Dohertyamplifier 130 may vary depending on the operation state of the peak amp133 according to power input to the Doherty amplifier 130.

FIG. 4 is a view for describing a change of output load impedance of thedriving amplifier 110 by the matching circuit 121 and the phaseadjusting unit 122 provided at the next stage of the driving amplifier110.

A point denoted by a reference number 401 in FIG. 4 represents anexample of output load impedance at low power, and a point denoted by areference number 402 in FIG. 4 represents an example of output loadimpedance at high power.

As illustrated in FIG. 4, by optimizing the matching circuit 121 and thephase adjusting unit 122 based on a change of input impedance of theDoherty amplifier 130, output load impedance of the driving amplifier110 may change depending on low power and high power. By changing outputload impedance of the driving amplifier 110, it is possible to increaseoutput power upon saturation of the driving amplifier 110 and to achievehigh efficiency at back off power.

Also, by changing output load impedance of the driving amplifier 110using the matching circuit 121 and the phase adjusting unit 122, theoutput load impedance of the driving amplifier 110 may vary according toa change in magnitude of power.

FIG. 5 is a graph showing high-frequency characteristics of the drivingamplifier 110 included in the amplifier circuit 100, according to anembodiment of the present disclosure.

In the graph shown in FIG. 5, the horizontal axis represents outputpower Pout [dBm], and the vertical axis represents Power AddedEfficiency (PAE) [%].

A reference number 501 of FIG. 5 corresponds to high-frequencycharacteristics of the driving amplifier 110 when impedance matching isdone to increase output power upon saturation, a reference number 502 ofFIG. 5 corresponds to high-frequency characteristics of the drivingamplifier 110 when impedance matching is done to obtain high efficiencyat back off power, and a reference number 503 of FIG. 5 corresponds tohigh-frequency characteristics of the driving amplifier 110 subject toimpedance matching by the matching circuit 121 and the phase adjustingunit 122 provided at the next stage of the driving amplifier 110.

As shown in FIG. 5, if output load impedance of the driving amplifier110 is adjusted by the matching circuit 121 and the phase adjusting unit122 provided at the next stage of the driving amplifier 110, it ispossible to obtain frequency characteristics capable of achieving highefficiency at back off power while increasing output power uponsaturation of the driving amplifier 110.

Also, by adjusting output load impedance of the driving amplifier 110using the matching circuit 121 and the phase adjusting unit 122 providedat the next stage of the driving amplifier 110, it is possible to reducedistortion and widen a dynamic range even when an operating pointgreatly wobbles. Particularly, it is preferred to increase efficiency atback off power for a modulation signal having a great PAPR, andreferring to the graph shown in FIG. 5, it is required to increaseefficiency at or near output power Pout of a point A. Accordingly, theamplifier circuit 100 according to the current embodiment can ensureefficient amplification particularly upon wireless transmission by amodulation signal having a great PAPR and requiring high efficiency atback off power.

Also, the amplifier circuit 100 according to the current embodiment asdescribed above has a configuration in which a plurality of amplifiersare connected in series to each other, and an amplifier of a final stageis a Doherty amplifier, however, an amplifier of the final stage is notlimited to a Doherty amplifier. That is, an amplifier of the final stagemay be any other amplifier whose input impedance varies depending onpower. For example, an envelope tracking amplifier may be used as anamplifier of the final stage.

The envelope tracking amplifier uses an envelope tracking method ofchanging a drain voltage of a Field Effect Transistor (FET) which is anamplification device of a power amplifier in synchronization with anenvelope of a signal. The envelope tracking amplifier can achieve highefficiency by reducing, when a signal level is low, a drain voltage tolower peak power of the amplifier and lowering back off power. When suchan envelope tracking amplifier is used as an amplifier of the finalstage, the amplifier circuit 100 may obtain frequency characteristicscapable of achieving high efficiency at back off power while increasingoutput power upon saturation of the driving amplifier 110.

Now, an example of a function configuration of a wireless base stationincluding the amplifier circuit 100 will be described.

FIG. 6 is a block diagram illustrating a function configuration of awireless base station including the amplifier circuit 100, according toan embodiment of the present disclosure.

The wireless base station illustrated in FIG. 6 is an example of acommunication apparatus.

Hereinafter, an example of a function configuration of a wireless basestation 10 including the amplifier circuit 100, according to anembodiment of the present disclosure, will be described with referenceto FIG. 6.

Referring to FIG. 6, the wireless base station 10 may include an inputinterface (I/F) 11, a digital circuit 12, a frequency converter 13, theamplifier circuit 100, an isolator 14, a Low-Pass Filter (LPF) 15, andan antenna 16.

The input interface 11 may be an interface for receiving signals. Theinput interface 11 may transfer a received signal to the digital circuit12. The digital circuit 12 may perform digital processing on thereceived signal, and transfer the signal subject to the digitalprocessing to the frequency converter 13.

The frequency converter 13 may convert the frequency of the signalreceived from the digital circuit 12, and transfer the signal whosefrequency has been converted to the amplifier circuit 100.

The isolator 14 may perform isolation on the signal amplified by theamplifier circuit 100, and transfer the signal subject to the isolationto the LPF 15. The LPF 15 may remove a noise component from the signalreceived from the isolator 14. The antenna 16 may output the signal fromwhich the noise component has been removed by the LPF 15.

By using the amplifier circuit 100 illustrated in FIG. 1 in the wirelessbase station 10 having the configuration illustrated in FIG. 6, thewireless base station 10 may obtain frequency characteristics capable ofachieving high efficiency at back off power while increasing outputpower upon saturation of the driving amplifier 110.

In FIG. 6, an example of a function configuration of a wireless basestation having the amplifier circuit 100, according to an embodiment ofthe present disclosure, is illustrated, however, the present disclosureis not limited to this example. For example, the amplifier circuit 100may be used in a Wireless Access Point (WAP) or a mobile phone in orderto obtain frequency characteristics capable of achieving high efficiencyat back off power while increasing output power upon saturation of thedriving amplifier 110.

As described above, according to an embodiment of the presentdisclosure, there is provided an amplifier capable of achieving highefficiency at back off power while maintaining high output power uponsaturation of a driving amplifier in a multistage amplifier in which aplurality of amplifiers are connected in series to each other.

The amplifier circuit 100 according to the embodiment of the presentdisclosure may include the impedance adjusting unit 120 for adjustingoutput load impedance of the driving amplifier 110, between the drivingamplifier 110 and the Doherty amplifier 130, as illustrated in FIG. 1.The impedance adjusting unit 120 may adjust impedance matching betweenthe driving amplifier 110 and the Doherty amplifier 130 according tooutput power from the driving amplifier 110.

Since the amplifier circuit 100 can variably optimize impedance matchingbetween the driving amplifier 110 and the Doherty amplifier 130according to output power from the driving amplifier 110, the amplifiercircuit 100 may obtain frequency characteristics capable of achievinghigh efficiency at back off power while increasing output power uponsaturation of the driving amplifier 110, as shown in FIG. 5.

FIG. 7 is a flowchart of an operation method of the amplifier circuit100, according to an embodiment of the present disclosure.

Referring to FIGS. 1 and 7, the impedance adjusting unit 120 may receivea signal from the driving amplifier 110, in step 701.

Then, the impedance adjusting unit 120 may adjust output load impedanceof the driving amplifier 110 according to a change of input impedance ofthe Doherty amplifier 130, in step 703. More specifically, the impedanceadjusting unit 120 may adjust output load impedance of the drivingamplifier 110 such that high-power load matching is done when high poweris output and high-efficient load matching is done at back off power.

Thereafter, the impedance adjusting unit 120 may transfer the adjustedoutput load impedance to the Doherty amplifier 130, in step 705.

Therefore, according to the embodiments of the present disclosure, thereare provided an amplifier circuit and a communication apparatus capableof achieving high efficiency at back off power while maintaining highoutput power when an amplifier of a driving stage is saturated in amultistage amplifier in which a plurality of amplifiers are connected inseries to each other.

While the disclosure has been shown and described with reference tocertain exemplary embodiments thereof, it will be understood by thoseskilled in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the disclosure asdefined by the appended claims and their equivalents.

The invention claimed is:
 1. An amplifier circuit comprising: at leasttwo amplifiers including a first amplifier and a second amplifier, thefirst amplifier preceding the second amplifier; and an impedanceadjusting unit disposed between the first amplifier and the secondamplifier, and configured to: adjust an output load impedance of thefirst amplifier, and adjust a phase of a signal output from the adjustedoutput load impedance, wherein the second amplifier is configured tochange input impedance according to an output power from the firstamplifier, and wherein the impedance adjusting unit is configured toadjust the output load impedance of the first amplifier according to achange of input impedance of the second amplifier.
 2. The amplifiercircuit according to claim 1, wherein the impedance adjusting unit isconfigured to adjust the output load impedance of the first amplifiersuch that high-power load matching is done based on a first amplifieroutputting high power and high-efficient load matching is done based ona first amplifier outputting back-off power.
 3. The amplifier circuitaccording to claim 1, wherein the impedance adjusting unit comprises amatching circuit configured to match the output load impedance of thefirst amplifier with the input impedance of the second amplifier.
 4. Theamplifier circuit according to claim 1, wherein the impedance adjustingunit comprises a phase adjusting unit configured to adjust a phase of asignal output from a matching circuit.
 5. The amplifier circuitaccording to claim 1, wherein the second amplifier is a Dohertyamplifier.
 6. The amplifier circuit according to claim 1, wherein thesecond amplifier is an envelope tracking amplifier.
 7. The amplifiercircuit according to claim 1, wherein the amplifier circuit is includedin a communication apparatus.
 8. A method of operating an amplifiercircuit, the method comprising: adjusting, at an impedance adjustingunit disposed between a first amplifier and a second amplifier, anoutput load impedance of the first amplifier; adjusting a phase of asignal output from the adjusted output load impedance; changing, by thesecond amplifier, an input impedance according to output power from thefirst amplifier; and adjusting, by the impedance adjusting unit, anoutput load impedance of the first amplifier according to a change ofinput impedance of the second amplifier, wherein the first amplifierprecedes the second amplifier.
 9. The method according to claim 8,wherein the impedance adjusting unit adjusts the output load impedanceof the first amplifier such that high-power load matching is done when afirst amplifier outputs high power and high-efficient load matching isdone when a first amplifier outputs back off power.
 10. The methodaccording to claim 8, wherein the impedance adjusting unit comprises amatching circuit configured to match the output load impedance of thefirst amplifier with the input impedance of the second amplifier. 11.The method according to claim 8, wherein the impedance adjusting unitcomprises a phase adjusting unit configured to adjust a phase of asignal output from a matching circuit.
 12. The method according to claim8, wherein the second amplifier is a Doherty amplifier.
 13. The methodaccording to claim 8, wherein the second amplifier is an envelopetracking amplifier.
 14. The method according to claim 8, wherein theamplifier circuit is included in a communication apparatus.
 15. Awireless communication apparatus comprising: a first amplifier; a secondamplifier, the first amplifier preceding the second amplifier; and animpedance adjusting unit disposed between the first amplifier and thesecond amplifier, and configured to: adjust an output load impedance ofthe first amplifier, and adjust a phase of a signal output from theadjusted output load impedance, wherein the second amplifier isconfigured to change input impedance according to an output power fromthe first amplifier, and wherein the impedance adjusting unit isconfigured to adjust the output load impedance and a phase of the firstamplifier according to a change of input impedance of the secondamplifier.
 16. The wireless communication apparatus according to claim15, wherein the impedance adjusting unit is configured to adjust theoutput load impedance of the first amplifier such that high-power loadmatching is done based on a first amplifier outputting high power andhigh-efficient load matching is done based on a first amplifieroutputting back-off power.
 17. The wireless communication apparatusaccording to claim 15, wherein the impedance adjusting unit comprises amatching circuit configured to match the output load impedance of thefirst amplifier with the input impedance of the second amplifier. 18.The wireless communication apparatus according to claim 15, wherein theimpedance adjusting unit comprises a phase adjusting unit configured toadjust a phase of a signal output from a matching circuit.
 19. Thewireless communication apparatus according to claim 15, wherein thesecond amplifier is a Doherty amplifier.
 20. The wireless communicationapparatus according to claim 15, wherein the second amplifier is anenvelope tracking amplifier.